Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. By a wide margin the most commonly employed photographic emulsions are silver bromide and bromoiodide emulsions. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
In almost every instance tabular grain emulsions have been formed by introducing two or more parallel twin planes into octahedral grains during their preparation. Regular octahedral grains are bounded by {111} crystal faces. The predominant feature of tabular grains formed by twinning are opposed parallel {111} major crystal faces. The major crystal faces have a three fold symmetry, typically appearing triangular or hexagonal.
The formation of tabular grain emulsions containing parallel twin planes is most easily accomplished in the preparation of silver bromide emulsions. The art has developed the capability of including photographically useful levels of iodide. The inclusion of high levels of chloride as opposed to bromide, alone or in combination with iodide, has been difficult. Silver chloride differs from silver bromide in exhibiting a much stronger propensity toward the formation of grains with faces lying in {100} crystalographic planes. To produce successfully a high chloride tabular grain emulsion by twinning, conditions must be found that favor both the formation of twin planes and {111} crystal faces. Further, after the emulsion has been formed, tabular grain morphological stabilization is required to avoid reversion of the grains to their favored more stable form exhibiting {100} crystal faces. When high chloride tabular grains having {111} major faces undergo morphological reversion to forms presenting {100} grain faces the tabular character of the grains is either significantly degraded or entirely destroyed and this results in the loss of the photographic advantages known to be provided by tabular grains.
Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) was the first to prepare in the presence of a 2-hydroaminoazine a high chloride emulsion containing tabular grains with parallel twin planes and {111} major crystal faces. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with an adsorbed aminoazaindene, preferably adenine, acting as a grain growth modifier.
Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II), significantly advanced the state of the art by preparing high chloride emulsions containing tabular grains with parallel twin planes and {111} major crystal faces using an aminoazaindene grain growth modifier and a gelatino-peptizer containing up to 30 micromoles per gram of methionine. Since the methionine content of a gelatino-peptizer, if objectionably high, can be readily reduced by treatment with a strong oxidizing agent (or alkylating agent, King et al U.S. Pat. No. 4,942,120), Maskasky II placed within reach of the art high chloride tabular grain emulsions with significant bromide and iodide ion inclusions prepared starting with conventional and universally available peptizers.
Maskasky I and II have stimulated further investigations of grain growth modifiers capable of preparing high chloride emulsions of similar tabular grain content. As grain growth modifiers, Tufano et al U.S. Pat. No. 4,804,621 employed 4,6-di(hydroamino)-pyrimidines lacking a 5-position amino substituent (a 2-hydroaminoazine species); Japanese patent application 03/116,133, published May 17, 1991, employed adenine (a 2-hydroaminoazine species) in the pH range of from 4.5 to 8.5; Takada et al U.S. Pat. No. 4,783,398 employed heterocycles containing a divalent sulfur ring atom; Nishikawa et al U.S. Pat. No. 4,952,491 employed spectral sensitizing dyes and divalent sulfur atom containing heterocycles and acyclic compounds; and Ishiguro et al U.S. Pat. No. 4,983,508 employed organic bis-quaternary amine salts.
In the foregoing patents there is little or no mention of stabilizing the tabular grain shape in the high chloride emulsions, since the continued presence of conditions favorable for stabilizing the {111} major faces of the tabular grains, usually the presence of a 2-hydroaminoazine, is assumed. Houle et al U.S. Pat. No. 5,035,992 specifically addresses the problem of stabilizing high chloride tabular grain emulsions prepared in the presence of a 2-hydroaminoazine (specifically 4,6-di(hydroamino)-pyrimidines lacking a 5-position amino substituent). Houle et al accomplished stabilization during tabular grain precipitation by continuously increasing the ratio of bromide to chloride being precipitated until the tabular grains were provided with stabilizing silver bromide shells. The Houle et al process is, of course, incompatible with producing a pure chloride emulsion, since at least some silver bromide must be included, and the process also has the disadvantage that the pyrimidine is left on the grain surfaces. Additionally, as shown in the Examples below, the grains remain morphologically unstable when their pH is lowered to remove the pyrimidine.
The emulsion teachings noted above either explicitly or implicitly suggest utilization of the emulsions with conventional grain adsorbed and unadsorbed addenda. A relatively recent summary of conventional photographic emulsion addenda is contained in Research Disclosure Vol. 308, December 1989, Item 308119. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD. England. While a wide variety of emulsion addenda can be adsorbed to grain surfaces, spectral sensitizing dyes and desensitizers (Res. Dis. Section IV) and antifoggants and stabilizers (Res. Dis. Section VI) are examples of photographically useful addenda that are almost always adsorbed to grain surfaces.